1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, a facsimile, and a multifunction peripheral having these functions. More particularly, the present invention relates to a configuration capable of executing a mode in which the developer conveyance by a developer bearing member is stopped while continuing the developer conveyance within a developing device.
2. Description of the Related Art
Conventionally, in electrophotographic type image forming apparatuses, a structure of the two-component development system using toner and carrier (two-component developer) is known. With the configuration of the two-component development system, a development screw as a conveyance member stirs and conveys toner and carrier, and charges toner within a development container of the developing device. Then, a developing sleeve as a developer bearing member carries and conveys toner and carrier, and, by using charged toner, develops an electrostatic latent image formed on a photosensitive drum to form a toner image thereon. With this configuration, for example, performing continuous printing of images having a low image ratio for a prolonged time period degrades the flowability of the developer. Accordingly, an immobile layer in which the developer is present being hard to move in the development container will be easily formed.
Accordingly, a structure has been discussed in which the height of the developer surface is temporarily changed by stopping the developer conveyance by a development sleeve while continuing the developer conveyance by conveying screws in the sheet-to-sheet interval, thus breaking down a developer's immobile layer (refer to Japanese Patent Application Laid-Open No. 2012-68390).
However, as with the above-described structure discussed in Japanese Patent Application Laid-Open No. 2012 -68390, when control is performed to stop the development sleeve in the sheet-to-sheet interval, the frictional force between the photosensitive drum and the intermediate transfer member changes. Normally, a small amount of fogging toner is constantly supplied onto the photosensitive drum with the rotation of the development sleeve . Fogging toner serves as a lubricant between the photosensitive drum and the intermediate transfer member, reducing the frictional force between the photosensitive drum and the intermediate transfer member.
On the other hand, when control is performed to stop the development sleeve, fogging toner is not supplied onto the photosensitive drum and therefore does not serve as a lubricant. As a result, the mechanical adhesion force between the photosensitive drum and the intermediate transfer member increases. If a transfer bias is applied between the photosensitive drum and the intermediate transfer member, the frictional force between the photosensitive drum and the intermediate transfer member increases because of an additional effect of the electrostatic force.
On the other hand, there is a transfer method called the slip transfer method in which image transfer is performed while generating a circumferential speed difference between the photosensitive drum and the intermediate transfer member. The slip transfer method is characterized in the effect of canceling error factors, such as a geometrical error, an outside diameter error, and an average angular velocity error of the photosensitive drum, appearing as out of color registration on the intermediate transfer member.
However, in a case where fogging toner disappears and the frictional force between the photosensitive drum and the intermediate transfer member increases as described above, the photosensitive drum or the intermediate transfer member, whichever is rotating at lower speed, tends to be taken around by the member rotating at higher speed. As a result, when the load on the member rotating at lower speed rapidly decreases to a fixed low level (nearly zero), the member may become out of speed control.
When a member becomes out of speed control in this way, rotating the development sleeve again to form an image may cause speed variation in the member rotating at lower speed, possibly causing image unevenness or out of color registration. Specifically, when fogging toner is supplied between the photosensitive drum and the intermediate transfer member by the rotation of the development sleeve, the frictional force between the photosensitive drum and the intermediate transfer member decreases, causing load variation in the member rotating at lower speed. At this timing, since the member rotating at lower speed is out of speed control, load variation causes speed variation. Then, the speed variation causes phenomena, such as a degraded latent image on the photosensitive drum and transfer unevenness, resulting in image unevenness and out of color registration.
When performing high-voltage control or sheet cassette changeover at the time of non-image formation, for example, in the sheet-to-sheet interval, the drive of the developing device may be stopped to prevent developer deterioration. Also in this case, the rotation of the development sleeve is stopped, and fogging toner is no longer supplied between the photosensitive drum and the intermediate transfer member, causing the same problem as that in the above-described case.